Phase Formation in Gels of the Apatite-Anorthite System

Phase formation in heat-treated composites containing anorthite gel and fluorapatite has been investigated. For the sol-gel synthesis TEOS, Al(NO3)3·9H2 O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O, NH4F and CaF2 were used as precursors. The gel mixtures were treated from 200 to 1250°C. It was found that the gels remain in an amorphous state up to 900°C. From X-ray diffraction structural analysis of anorthite gel-glasses heat treated at 600°C and 800°C it was found that a shift of the first maximum occurred from 1.85 to 1.77 Å, which could be interpreted as a transformation of the gel-glass structure with increasing temperature. During the heat treatment of the mixed gels from the apatite-anorthite system it was found that fluorapatite is present as a major crystal phase at 950°C. At 1250°C anorthite and gehlenite are the major crystalline phases.